1. Field of the Invention
This invention pertains to the spatial and angular modulation of electromagnetic waves. More particularly, this invention relates to optically controlling the spatial and angular modulation of electromagnetic waves incident upon a semiconductor material by optically changing the complex permittivity of the semiconductor material.
2. Description of the Prior Art
Light can change the complex refractive index n of a semiconductor material. Specifically, n=n (I) where n=n'+in" (where n' and in" are the real and imaginary parts respectively and i=.sqroot.-1) and I is the intensity of the optical wave. The mechanics of this phenomenon is based on fundamental Drude theory. See T. S. Moss, "Optical Properties of Semiconductors," Butterworths, London (1959).
The intensity I of an optical wave can change the complex refractive index of Si, GaAs, InGaAsP and other semiconductors in the microwave range (1 mm-1 cm) and infrared (IR) range (1.0.mu.- 100.mu.). See I. Shih, "Photo-Induced Complex Permittivity Measurements of Semiconductors" 477 SPIE 94 (1984) (microwave range) and B. Bennett, "Carrier-Induced Change in Refractive Index of InP, GaAs, and InGaAsP" 26 IEEE J. Quan. Elec. 113 (1990) (IR range) incorporated herein by reference.
The prior art shows light induced modulation of both the real and imaginary parts of the refractive index. The real part controls phase and the imaginary part controls amplitude of the modulated electromagnetic field. The real part is primarily responsible for changes in IR waves and the imaginary part for changes in millimeter waves (MMW). This effect is described by Drude theory and involves carrier induced changes in the complex permittivity of metals and semiconductors when illuminated by light. Light increases the density of free carriers in the material.
Based on this effect, devices which change the phase of lightwaves by illumination of semiconductors with other light have been developed. Specifically, optical phase modulators have been employed. In the state of the art, however, it is shown possible to modulate the material at only one point. This type of limited modulation is discussed in a recent article by Z. Y Cheng and C. S. Tsai "Optically Activated Integrated Optic Mach-Zender Interferometer on GaAs," 59 Appl. Phys. Lett. 1991. It would be beneficial to employ a device that can modulate an EM field at more than one point, particularly to modulate the material in two dimensions (2D) and potentially three dimensions (3D).
At the same time, optically controlled spatial light modulators (SLM) based on semiconductor materials have been used. In optically addressed SLMs, the semiconductor plays a transport role, such that changes in the semiconductor material affect an adjacent layer of electro-optic (EO) material which in turn affects an EM wave propagating through the EO material. The effectiveness of this type of modulator is low. It should be mentioned that these devices are limited to controlling the visible range only.
These SLM devices transmit or project some 2D pattern which can be transmitted through an optical wave. Other types of devices that are of interest, however, transmit EM waves in a particular direction in the microwave region without moving parts. Such devices are called phased array antennas.
A phased array is a network of radiating elements, each of which is usually non-directive but whose cooperative radiation pattern is a highly directed beam because constructive interference occurs between radiating elements. Whereas previous radar antennas had to be mechanically steered for beampointing, the phased array antenna achieves the same effect electronically by individually changing the phases of the signals radiating from each element. Narrow angular band beams can be formed by simply driving each element of the array with an appropriately phased signal. Moreover, electronic steering is much faster and more agile than mechanical beam steering and can form several beam lobes and nulls to facilitate multiple target tracking or other functions such as anti-jamming.
The flexibility of electronic steering afforded by phased array radars, however, comes at the cost of individual control of each element. The N elements of the antenna are driven with the same signal but each with a different phase. In practice, a single signal is equally split into N signals to feed the elements, and a phase shifting network, such as those using ferrites or diodes, is provided for individual phase control of each element. For large arrays (i.e., N&gt;100), the complexity of the power splitting network and the cost of providing N phase shifters can become quite high, not to mention the bulkiness of the necessary waveguide plumbing. Moreover, for very large arrays, the computation required to calculate the array phase distribution for a desired radiation pattern is a serious burden. These constitute the most serious drawbacks of conventional phased array radar systems.
Phased array antenna theory is based on Fourier optics in general and the theory of diffraction gratings in particular. It is well known from Fourier optics that the optical beam is diffracted in a particular direction if the phase difference between the particular optical rays is a multiple of the wavelength of the optical beam.
According to FIG. 1A, the phase synchronized condition has the form m.lambda.=.LAMBDA. sin .theta., where .LAMBDA. is the grating constant, .theta. is the angle of diffraction, and m is the integers 0, .+-.1, .+-.2 . . . . From this equation we obtain the following equation ##EQU1## which is the well known grating equation. If electrically controlled, a phase shift of m.lambda. can be introduced between different antennas thereby causing constructive interference in one direction, which results in antenna directionality. This effect can be used in both a transmitter and a receiver.
Exactly the same principle is used in conventional phased array antennas where illuminated points of the gratings are replaced by elementary antennas as in FIG. 1B. See M. I. Skolnik, "Introduction to Radar Systems", McGraw Hill New York (1980) incorporated herein by reference.
Using Equation 1 two basic disadvantages of phased array antenna systems are made apparent: (1) the periodic structure has a discrete point-type profile. This means that many diffraction orders are generated; only one order is desired, and the remaining orders reduce efficiency of the system; (2) the number of elementary antennas is limited by size and complexity. As the frequency of the microwaves increase (beyond 60 GHz), the density of packaging of individual elements and phase shifters limits the feasibility of such an antenna. Also, having individual emitters fixed in space precludes the antenna from being used for different frequencies. At the receiver end, such an antenna has limited bandwidth capability (due to the fixed elements).
For IR beam steering the packaging of individual phase shifters is virtually impossible and an electronically controlled spatial light detector is limited to very narrow angular bandwidth. See F. Vasey et al., "Electro-optic AlGaAs Spatial Light Deflector/Modulator Based on a Grating Phased Array" 58 Appl. Phys. Lett. 2874 (1991) incorporated herein by reference.